Frequency enhanced impedance dependent power control for multi-frequency RF pulsing

ABSTRACT

Methods for processing a substrate in a plasma processing chamber employing a plurality of RF power supplies. The method includes pulsing at a first pulsing frequency a first RF power supply to deliver a first RF signal between a high power state and a low power state. The method further includes switching the RF frequency of a second RF signal output by a second RF power supply between a first predefined RF frequency and a second RF frequency responsive to values of a measurable chamber parameter. The first RF frequency and the second RF frequencies and the thresholds for switching were learned in advance during a learning phase while the first RF signal pulses between the high power state and low power state at a second RF frequency lower than the first RF frequency and while the second RF power supply operates in different modes.

PRIORITY CLAIM

This application is a Continuation application of U.S. application Ser.No. 13/621,759, filed on Sep. 17, 2012, entitled Frequency EnhancedImpedance Dependent Power Control for Multi-Frequency RF Pulsing, whichclaims priority under 35 USC. 119(e) to a commonly-owned provisionalpatent application entitled “Frequency Enhanced Impedance DependentPower Control For Multi-Frequency RF Pulsing”, U.S. Application No.61/602,040, filed on Feb. 22, 2012 by John C. Valcore Jr. et al., all ofwhich are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Plasma processing has long been employed to process substrates (e.g.,wafer or flat panels or other substrates) to create electronic devices(e.g., integrated circuits or flat panel displays). In plasmaprocessing, a substrate is disposed in a plasma processing chamber,which employs one or more electrodes to excite a source gas (which maybe an etchant source gas or a deposition source gas) into a plasma forprocessing the substrate. The electrodes may be excited by one or moreRF signals, which may be furnished by one or more RF generators, forexample.

In some plasma processing systems, multiple RF signals, some of whichmay have the same or different RF frequencies, may be provided to one ormore electrodes to generate plasma. In a typical capacitively-coupledplasma processing system, for example, one or more RF signals may beprovided to the top electrode, the bottom electrode, or both in order togenerate the desired plasma.

In some applications, one or more of the RF signals may be pulsed. Forany given RF signal, RF pulsing involves alternately changing the RFsignal between a high power set point and a low power set point. When anRF signal from an RF generator (e.g., RF_GEN1) is pulsed, the amount ofRF power delivered by RF_GEN1 to the plasma load changes dependingwhether the RF signal is pulsed high or pulsed low. The changes in theRF power level delivered to the plasma load result in changes in theplasma impedance. For example, the plasma impedance may be at one levelwhen RF generator RF_GEN1 is pulsed high, and at another level when RFgenerator GF_GEN1 is pulsed low.

If other RF generators have their frequencies tuned to maximize theirpower delivery based on the plasma impedance that exists during the highpulse of the RF signal from RF generator RF_GEN1, these RF frequencieswill likely result in inefficient power delivery when the plasmaimpedance changes due to the fact that the RF power level delivered byRF generator RF_GEN1 has changed when the RF signal from RF_GEN1 pulseslow, for example.

To further elaborate on the frequency tuning aspect, a modern RFgenerator may self-tune its RF frequency in order to more properly matchthe output impedance of that RF generator to the plasma load. As theterm is employed herein, the independently pulsing (IP) RF signal refersto the RF signal that pulses independently of other RF signals. Suchindependently pulsing RF signal may pulse in response to a command fromthe tool host or another control circuit for example. A dependent RFsignal is an RF signal that tunes or changes its RF frequency in orderoptimize its power delivery to the plasma load in response to thepulsing of the IP RF signal.

In the prior art, the dependent RF generator that provides the dependentRF signal may sweep through multiple frequencies during its frequencytuning process (such as in response to a plasma impedance change eventcaused by the pulsing of the IP RF signal). The dependent RF generatormay monitor the forward power and reflected power during the frequencyself-tuning process to determine the RF frequency that most efficientlydelivers power to the plasma load as it sweeps through differentfrequencies.

In theory, the prior art self-tuning works adequately for certainapplications. However, the RF signal pulsing frequency specified bymodern processes is generally too fast (e.g., 10 KHz or faster) forself-tuning feature of dependent RF generators to keep up. This isbecause, in part, multiple samples are needed for frequency self-tuning,which requires the tuning/dependent RF generator to operate atimpractically high frequencies in order to perform acceptable frequencytuning.

If the RF frequency of a dependent RF generator is not tuned quicklyenough to adapt to the changing plasma impedance (such as the plasmaimpedance change following a high-to-low or low-to-high transition ofthe IP RF signal), power delivery by that dependent RF generatorsremains inefficient until the dependent RF signal has its frequencytuned. The longer the dependent RF generator takes to tune itsfrequency, the longer the time period during which power delivery bythat dependent RF generator is non-optimal.

In view of the forgoing, there are desired improved methods andapparatus for ensuring that the RF frequencies of the dependent RFgenerators can quickly react to changes in the plasma impedance causedby IP RF signal pulsing.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements and in which:

FIG. 1 shows, in accordance with one or more embodiments of theinvention, a forward power versus time diagram of two RF signals, a 2MHz signal and a 60 MHz signal during the learning period.

FIG. 2 shows, in accordance with one or more embodiments of theinvention, the steps of the learning process.

FIG. 3 shows, in accordance with one or more embodiments of theinvention, a diagram of normalized RF parameters versus time forimplementing the rapid frequency tuning by the dependent RF generatorsfor optimal production time power delivery during IP RF signal pulsing.

FIG. 4 shows, in accordance with one or more embodiments of theinvention, the steps for implementing the rapid frequency tuning by thedependent RF generators for optimal power delivery during pulsing.

DETAILED DESCRIPTION OF EMBODIMENTS

The present invention will now be described in detail with reference toa few embodiments thereof as illustrated in the accompanying drawings.In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art, that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process steps and/or structureshave not been described in detail in order to not unnecessarily obscurethe present invention.

Various embodiments are described herein below, including methods andtechniques. It should be kept in mind that the invention might alsocover articles of manufacture that includes a computer readable mediumon which computer-readable instructions for carrying out embodiments ofthe inventive technique are stored. The computer readable medium mayinclude, for example, semiconductor, magnetic, opto-magnetic, optical,or other forms of computer readable medium for storing computer readablecode. Further, the invention may also cover apparatuses for practicingembodiments of the invention. Such apparatus may include circuits,dedicated and/or programmable, to carry out tasks pertaining toembodiments of the invention. Examples of such apparatus include ageneral-purpose computer and/or a dedicated computing device whenappropriately programmed and may include a combination of acomputer/computing device and dedicated/programmable circuits adaptedfor the various tasks pertaining to embodiments of the invention.

Embodiments of the invention relate to methods and apparatus forenabling RF generators to quickly change the RF frequency of their RFsignals to match the plasma load condition when the independentlypulsing (IP) RF signal pulses. In one or more embodiments of theinvention, each dependent generator undergoes a learning process duringa learning period, during which its own optimal tuned RF frequency islearned for the plasma impedance conditions that exist when the IP RFsignal pulses high and when the IP RF signal pulses low. The learningperiod represents a time duration during which the high pulse durationof the IP RF signal is artificially extended to give the dependent RFgenerator time to converge to an optimal tuned RF frequency for the casewhen the IP RF signal is pulsed high. Furthermore, the low pulseduration of the IP RF signal is also artificially extended to give thedependent RF generator time to converge to another optimal tuned RFfrequency for the case when the IP RF signal is pulsed low. Embodimentsof the invention also involve techniques for ascertaining thresholdvalues of a measurable plasma parameter for deciding during production(non-learning) time when the IP RF signal has made its transition fromhigh-to-low or from low-to-high. One threshold value signifies ahigh-to-low transition of the IP RF signal and another threshold valuesignifies the low-to-high transition of the IP RF signal.

During production use (such as during plasma processing of substrates),each dependent RF generator may monitor the measurable plasma parameter,which threshold values are learned during the aforementioned learningperiod. If the measurable plasma parameter value exceeds either of thethreshold values, a transition in the IP RF signal power level is deemedto have occurred and the dependent RF generator switches to theappropriate learned optimal tuned RF frequency value depending onwhether the IP RF signal is pulsing high-to-low or low-to-high.

Note that embodiments of the invention employ the self-tuning feature inthe dependent RF generators only during the learning time, whichgenerally precedes the production time (i.e., when the plasma system isactually employed for processing substrates during production). Once theoptimal tuned RF frequencies and the measurable plasma parameterthresholds are ascertained for different pulsing conditions of the IP RFsignal, each dependent RF generator simply flips from one previouslylearned optimal tuned RF frequency value for that dependent RF generatorto another previously learned optimal tuned RF frequency value for thatdependent RF generator during production time. The dependent RFgenerator changes its learned optimal tuned RF frequency when it detectsthat the monitored measureable plasma parameter made an excursion beyondthe previously learned thresholds. In this manner, the time consumingprocess of frequency self-tuning by the dependent RF generators iseliminated during production time, and the dependent RF generators maymore quickly optimize its power delivery to the plasma during productiontime when the IP RF signal pulses.

The features and advantages of embodiments of the present invention maybe better understood with reference to the figures and discussions thatfollow.

FIG. 1 shows, in accordance with one or more embodiments of theinvention, a forward power versus time diagram of two RF signals, a 2MHz signal and a 60 MHz signal during the learning period. Generallyspeaking, the learning period is performed once for each recipe to learnthe optimal tuned RF values and the threshold values or performed onlyonce in a while to account for, for example, chamber drift. In thisexample, the 2 MHz signal is the independently pulsing (IP) RF signaland the 60 MHz signal represents the dependent RF signal that tunes itsRF frequency to optimize power delivery when the 2 MHz RF signal pulses.Although only one dependent RF generator (e.g., 60 MHz) is discussed inconnection with FIG. 1, it should be understood that there may bemultiple dependent RF generators, each of which may undergo the samelearning process to ascertain its own optimal tuned RF frequencies andthresholds when the IP RF signal pulses. FIG. 1 may be better understoodwhen studied in conjunction with the flowchart of FIG. 2, which shows indetails the steps of the learning process (starting at step 202).

At point 102, the IP RF generator (e.g., 2 MHz generator) is pulsed highto its high power set point. In the example of FIG. 1, the high powerset point for the 2 MHz IP RF generator is 6 kW. This is also shown instep 204 of FIG. 2. Further, the dependent RF generator (e.g., the 60MHz generator) is set to its frequency self-tuning mode to allow thedependent RF generator to converge to the optimal RF frequency for powerdelivery when the IP RF signal (104) is pulsed high. This is also shownin step 204 of FIG. 2.

To elaborate, modern RF generators monitor many plasma chamberparameters and adjust their own parameters to maximize power delivery.One common measure of power delivery efficiency is the relationshipbetween forward power and reflected power, also known as gamma. If thegamma value is zero, power delivery is deemed highly efficient. If thegamma value is 1, power delivery is deemed highly inefficient. Theforward and reflected powers may be monitored using the RF generatorpower sensors, for example. In frequency-tuned RF generators, the RFgenerator then tunes its RF signal frequency to minimize gamma, therebymaximizing power delivery efficiency.

The IP RF signal of 2 MHz is pulsed high during the period betweenpoints 102 and 106. This high pulse duration of the IP RF signal isgreatly extended during learning time (from tenths of seconds up tomultiple of seconds, in one or more embodiments) relative to the true IPRF signal high pulse duration employed during production time forsubstrate processing (the true pulsing period of the IP RF signal isdetermined by the pulsing frequency specified in the plasma processingrecipe during production time). This artificially extended high pulseduration of about 3 seconds (from point 102 to point 106) gives thedependent RF generator (e.g., the 60 MHz generator) enough time tooptimally tune its frequency to maximize power delivery efficiency forthe plasma impedance condition that exists when the IP RF signal ispulsed high. Note that the artificially extended high pulse duration ofthe IP RF signal during the learning time enables this optimal frequencytuning by the dependent RF generator even if the true pulsing period ofthe IP RF signal during production time is too short for properdependent RF generator frequency tuning.

The 60 MHz dependent RF generator tunes to 61.3 MHz for a gamma value of0.04 when the 2 MHz IP RF signal pulses high. This optimal tuned RFfrequency of 61.3 MHz (IDPC_FREQ1 frequency) for the 60 MHz dependent RFgenerator is then recorded (step 206 and step 208 of FIG. 2). This 61.3MHz value represents the optimal RF frequency for the 60 MHz dependentRF signal when the 2 MHz IP RF signal pulses high. The gamma value of0.04 verifies that power delivery is efficient at this optimal tuned RFfrequency for the 60 MHz dependent RF generator.

The 60 MHz generator is then operated in the fixed frequency modewhereby its RF frequency is not allowed to tune. Instead, the 60 MHzgenerator is forced to operate at the aforementioned 61.3 MHz optimaltuned RF frequency and the 2 MHz IP RF signal transitions from its highpower set point to its low power set point (from 106 to 108). This canbe seen in step 210 of FIG. 2. Although the low power set point for the2 MHz RF signal is zero in the example of FIG. 2, such is not arequirement. The low power set point may be any power level setting thatis lower than the high power set point of 6 kW, for example.

After the 2 MHz IP RF signal pulses low (after point 108), the previousoptimal tuned RF frequency of 61.3 MHz is no longer the most efficientRF frequency for power delivery by the 60 MHz RF generator. This isbecause the plasma impedance has changed when the 2 MHz independentlypulsing RF signal pulses low to deliver a lower amount of RF power tothe plasma. The inefficiency is reflected in the gamma value of 0.8,which is detected by the power sensors of the 60 MHz dependent RFgenerator. This gamma value of 0.8 is recorded (step 212 of FIG. 2) andmay be set as the IDPC_Gamma1 threshold (step 214 of FIG. 2) in one ormore embodiments. During production time, as the IP RF signal is pulsedhigh and the 60 MHz RF signal is at 61.3 MHz (the first optimal tuned RFfrequency for the 60 MHz RF generator when the IP RF signal is pulsedhigh) and the IDPC_Gamma1 threshold is subsequently encountered, the 60MHz dependent RF generator would know that the 2 MHz IP RF signal hasjust transitioned from high to low.

In one or more embodiments, the IDPC_Gamma1 threshold can be adjustedfor sensitivity by a Threshold1_Adjust value. For example, it may bedesirable to set (step 214 of FIG. 2) the IDPC_Gamma1 threshold at 0.7instead of 0.8 (i.e., slightly below the true gamma value that existsdue to the high-to-low transition of the 2 MHz IP RF signal) to increasethe high-to-low detection sensitivity by the power sensors of the 60 MHzdependent RF generator. In this example, the Threshold1_Adjust valuewould be (−0.1), and the IDPC_Gamma1 threshold of 0.7 is the sum of thetrue gamma value (0.8) and the Threshold1_Adjust value of −0.1.

Once the IDPC_Gamma1 value is obtained, the 60 MHz dependent RFgenerator is set to the frequency self-tuning mode (step 216) to enablethe 60 MHz dependent RF generator to determine the optimal tuned RFfrequency for power delivery when the 2 MHz IP RF signal pulses low.Again, the low pulse of the 2 MHz IP RF signal is artificially extended(between points 108 and 110 of FIG. 1) to enable both the ascertainmentof the IDPC_Gamma1 value and to permit the 60 MHz dependent RF generatorto self-tune to an optimal RF frequency for power delivery during thelow pulse of the 2 MHz IP RF signal.

Once the 60 MHz dependent RF generator tunes to the optimal RF frequency(60.5 MHz in the example of FIG. 1) for power delivery during the lowpulse of the 2 MHz IP RF signal, the new optimal tuned RF frequency ofthe 60 MHz dependent RF generator is recorded (step 218 and 220 of FIG.2).

After the 60 MHz dependent RF generator has tuned to its optimal RFfrequency value of 60.5 MHz (IDPC_FREQ2 frequency) for the low pulse ofthe 2 MHz IP RF signal, the 60 MHz dependent RF generator is set tooperate in the fixed frequency mode (step 222) at the IDPC_FREQ2frequency of 60.5 MHz and the 2 MHz IP RF generator is allowed to pulsehigh (transition from 110 to 112). This can also be seen in step 222 ofFIG. 2.

After the 2 MHz IP RF signal pulses high (after point 112), the previousoptimal tuned RF frequency of 60.5 MHz (IDPC_FREQ2 frequency) is nolonger the most efficient RF frequency for power delivery by the 60 MHzRF generator. This is because the plasma impedance has changed when the2 MHz independently pulsing RF signal pulses high to deliver a higheramount of RF power to the plasma. The inefficiency is reflected in thegamma value of 0.78, which is detected by the power sensors of the 60MHz dependent RF generator. This gamma value of 0.78 is recorded (step224 of FIG. 2) and may be set as the IDPC_Gamma2 threshold (step 226 ofFIG. 2) in one or more embodiments. During production time as the IP RFsignal is pulsed low and the 60 MHz RF signal is at 60.5 MHz (the secondoptimal tuned RF frequency for the 60 MHz RF generator when the IP RFsignal is pulsed low) and the IDPC_Gamma2 threshold is subsequentlyencountered, the 60 MHz dependent RF generator would know that the 2 MHzIP RF signal has just transitioned from low to high.

In one or more embodiments, the IDPC_Gamma2 threshold can be adjustedfor sensitivity by a Threshold2_Adjust value. For example, it may bedesirable to set (step 226 of FIG. 2) the IDPC_Gamma2 threshold at 0.75instead of 0.78 (i.e., slightly below the true gamma value that existsdue to the low-to-high transition of the 2 MHz IP RF signal) to increasethe low-to-high detection sensitivity by the power sensors of the 60 MHzdependent RF generator. In this example, the Threshold2_Adjust valuewould be (−0.03), and the IDPC_Gamma2 threshold of 0.75 is the sum ofthe true gamma value (0.78) and the Threshold2_Adjust value of −0.03.

The two optimal tuned RF frequencies values (61.3 MHz and 60.5 MHz) andthe two gamma threshold values (IDPC_Gamma1 and IDPC_Gamma2) are thenemployed during production time when the 2 MHz is allowed to pulsenormally per the plasma processing recipe and the 60 MHz dependent RFgenerator simply flips back and forth between the two previously learnedoptimal tuned RF frequencies (61.3 MHz and 60.5 MHz) when its powersensors detects that the gamma value has exceeded the thresholds(IDPC_Gamma1 and IDPC_Gamma2). The production time frequency tuning bythe dependent RF generator is discussed in connection with FIGS. 3 and 4below.

FIG. 3 shows, in accordance with one or more embodiments of theinvention, a diagram of normalized RF parameters versus time forimplementing the rapid frequency tuning by the dependent RF generatorsfor optimal production time power delivery during IP RF signal pulsing.FIG. 3 may be better understood when studied in conjunction with theflowchart of FIG. 4, which details the steps for implementing the rapidfrequency tuning by the dependent RF generators for optimal powerdelivery during pulsing (starting at step 402).

At point 302, the 2 MHz IP RF generator is pulsed high and the 60 MHzdependent RF generator is set to its previously learned optimal RFfrequency of IDPC_FREQ1 (e.g., 61.3 MHz) or allowed to self-tune to thisoptimal RF frequency of IDPC_FREQ1. This is seen in step 404 of FIG. 4.Thereafter, the dependent RF generator operates in the rapid frequencytuning mode.

In the example of FIG. 3, the 2 MHz IP RF signal pulses at a pulsingfrequency of 159.25 Hz with a 50% duty cycle (the duty cycle can vary ifdesired depending on recipes) between a high power set point of 6 kW anda low power set point of 0 kW (the 0 kW is not a requirement and theinvention works equally well if the low power set point is non-zero).The 60 MHz dependent RF generator provides power at a power set point of900 W.

While the 60 MHz dependent RF generator delivers power to the plasmaload, it also monitors the gamma value via its power sensors (steps 406and 408 of FIG. 4). At point 304, the 2 MHz IP RF signal pulses low.Shortly after this high-to-low transition, the gamma value measured bythe 60 MHz dependent RF generator spikes from around 0.04 to around 0.8(point 306 to point 308). If the IDPC_Gamma1 threshold is set at to tripat, for example, 0.7, the excursion by the detected gamma value (branchYES of step 408) would cause the 60 MHz RF generator to flip from onepreviously learned optimal tuned RF frequency value (IDPC_FREQ1frequency of 61.3 MHz) to the other previously learned optimal tuned RFfrequency value (IDPC_FREQ2 frequency of 60.5 MHz). This is seen in step410 of FIG. 4. This rapid tuning of the 60 MHz dependent RF generatorfrom 61.3 MHz to 60.5 MHz in response to the high-to-low transition ofthe 2 MHz IP RF signal brings the measured gamma value down to 0.05(from point 310 to point 312).

At point 314, the 2 MHz IP RF signal pulses from low to high (314 to322). Shortly after this low-to-high transition, the gamma valuemeasured (steps 412 and 414 of FIG. 4) by the 60 MHz dependent RFgenerator spikes from around 0.05 to around 0.78 (point 314 to point316). If the IDPC_Gamma2 threshold is set at to trip at, for example,0.75, the excursion by the detected gamma value (YES branch of step 414of FIG. 4) would cause the 60 MHz RF generator to flip from thepreviously learned optimal tuned RF frequency value (IDPC_FREQ2frequency of 60.5 MHz) to the other previously learned optimal tuned RFfrequency value (IDPC_FREQ1 frequency of 61.3 MHz). This is seen in step404 of FIG. 4. This rapid tuning of the 60 MHz dependent RF generatorfrom 60.5 MHz to 61.3 MHz in response to the low-to-high transition ofthe 2 MHz IP RF signal brings the measured gamma value down to 0.04(from point 318 to point 320).

It should be noted that the time scale of FIG. 3 (production time)reflects a faster time scale than that of FIG. 1 (learning time). Thisis the case when, as mentioned, the high duration and the low durationof the IP RF pulses are artificially extended during learning time topermit the dependent RF generators to self-tune to the optimal tune RFfrequencies for learning purposes. During production time, suchself-tuning is not necessary since the dependent RF generator operatesessentially as a state machine and flips from one learned optimal RFfrequency to another learned optimal RF frequency as it detects thehigh-to-low transition of the IP RF signal (by comparing the measuredgamma value versus IDPC_Gamma1 threshold and by knowing the previousstate of the IP RF signal prior to the detection of the gamma excursion)and the low-to-high transition of the IP RF signal (by comparing themeasured gamma value versus the IDPC_Gamma2 threshold and by knowing theprevious state of the IP RF signal prior to the detection of the gammaexcursion)

It should be noted at this point that although only one dependent RFgenerator is discussed in connection with the examples of FIGS. 1-4herein, multiple dependent RF generators may learn their own optimaltuned RF frequencies and their own IDPC Gamma thresholds in the samemanner to enable them to rapidly tune their RF frequencies for maximumpower delivery efficiency during production time.

As can be appreciated from the foregoing, embodiments of the inventionfacilitate rapid frequency tuning by the dependent RF generators duringproduction time. The trade-off is the time spent learning the optimaltune frequency values and the IDPC Gamma thresholds. However, thislearning time occurs only once for the recipe and is typically performedprior to production substrate processing (i.e., production time).

By operating the dependent RF generators as simple state machines andeliminating the frequency self-tuning steps by the dependent RFgenerators during production time, optimal power delivery is achievedearly with every transition of the IP RF signal pulse (since it is muchless time consuming to flip from one previously learned optimal RFfrequency value to another previously learned optimal RF frequency valuethan to iterate through a range of frequencies to find an optimal tuneRF frequency during each transition of the IP RF signal). Furthermore,in the case when the pulsing frequency of the IP RF signal is simply toofast during production time for the dependent RF generators to frequencytune by the self-tuning process, the state-machine-like manner ofoperation during production time makes frequency tuning possible tomaximize power delivery efficiency.

While this invention has been described in terms of several preferredembodiments, there are alterations, permutations, and equivalents, whichfall within the scope of this invention. It should also be noted thatthere are many alternative ways of implementing the methods andapparatuses of the present invention. Although various examples areprovided herein, it is intended that these examples be illustrative andnot limiting with respect to the invention.

What is claimed is:
 1. A method for processing a substrate in a plasmaprocessing system having a processing chamber coupled to a plurality ofRF power supplies for sustaining a plasma within said processingchamber, comprising: operating a first RF power supply to deliver afirst RF signal between a high power state and a low power state to theprocessing chamber, wherein said first RF signal has a first frequency;and operating a second RF power supply of said plurality of RF powersupplies in a fixed frequency mode so that the second RF power supply isnot permitted to self-tune a frequency of a second RF signal output tothe processing chamber by said second RF power supply, wherein saidsecond RF signal operates with a first fixed RF frequency value and asecond fixed RF frequency value, wherein the first and second fixed RFfrequency values are learned earlier during a learning phase when saidfirst RF signal is respectively at the high power state and said lowpower state.
 2. The method of claim 1, wherein in the learning phase,the second RF power supply operates in a frequency self-tuning mode. 3.The method of claim 1 wherein said first RF power supply is anindependently pulsing (IP) RF power supply.
 4. The method of claim 3,wherein during the learning phase, the first fixed RF frequency valueand the second fixed RF frequency value are identified by self-tuningduring which the first and second fixed RF frequency values are selectedbased on a at least one measurable chamber parameter from saidprocessing chamber.
 5. The method of claim 4, wherein the at least onemeasurable chamber parameter is a gamma value, which is a relationshipbetween forward power and reflected power.
 6. The method of claim 5,further comprising, comparing the gamma value with pre-definedthresholds.
 7. The method of claim 6 whereby said pre-defined thresholdsare acquired during said learning phase.
 8. A method for operating aplasma processing system, comprising: executing a learning phase, saidlearning phase comprising, a) operating a first RF power supply tochange a power level of a first RF signal between a low power state to ahigh power state, wherein said first RF power supply operates at a firstpulsing frequency, b) operating a second RF power supply in aself-tuning mode so that the second RF power supply self-tunes afrequency of a second RF signal output by said second RF power supply toidentify a first RF frequency value when said first RF signal is in saidhigh power state, and c) operating said second RF power supply in saidself-tuning mode so that the second RF power self-tunes the frequency ofthe second RF signal output by said second RF power supply to identify asecond RF frequency value when said first RF signal is in its low powerstate.
 9. The method of claim 8, further comprising, executing aproduction phase, said production phase comprising, d) operating said RFpower supply to alternate said power level of a first RF signal betweensaid low power state to said high power state, wherein said operating insaid production phase is performed at a second pulsing frequency fasterthan said first pulsing frequency, and e) alternating said frequency ofsaid second RF power supply between said first RF frequency value andsaid second RF frequency value while operating said second RF powersupply in a fixed frequency mode.
 10. The method of claim 9, whereinsaid second RF power supply, in said fixed frequency mode, is notpermitted to self-tune said frequency of said second RF signal in themanner that said second RF power supply self-tunes said frequency ofsaid second RF signal while in the self-tuning mode.
 11. The method ofclaim 8, wherein during the learning phase, the first RF frequency valueand the second RF frequency value are identified by self-tuning duringwhich the first and second RF frequency values are selected based on aat least one measurable chamber parameter from said processing chamber.12. The method of claim 11, wherein said measurable plasma parameterrepresents gamma.
 13. The method of claim 12, wherein gamma is arelationship between forward power and reflected power.
 14. The methodof claim 12, further comprising, comparing gamma with pre-definedthresholds.
 15. The method of claim 14 wherein said pre-definedthresholds are acquired during said learning phase.